You cannot use the Glue tool to extend a filled object. Glue will only work with the open ends of an object. To convert a closed object into an open one, select the Scaplel tool, hold down the Ctrl key, and click on one of the corners of the object. You can now use glue to attach things where you separated the line. Also, make sure you are holding down the mouse button during the glue operation, releasing only once your the black glue bottle is over the point to attach to.